1. Field of the Invention
Embodiments of the present invention generally relate to cables for use in high temperature environments, such as downhole gas and oil well applications.
2. Description of the Related Art
With advancements in the area of fiber optic sensors for use in harsh environments, there is an increasing need for fiber optic cables compatible with the harsh environmental conditions present in downhole oil and gas well applications. FIG. 1 illustrates a schematic cross-sectional view of a wellbore 102, wherein a cable 110 is provided as a sensing device within the wellbore 102. Properties of downhole formations surrounding or otherwise adjacent the wellbore 102 may be monitored over time based on measurements made by the sensing device. Further, hydrocarbon production may be controlled or reservoirs 108 may be managed based on the downhole formation properties.
FIG. 1A illustrates a partial sectional side view of the cable 110. Cables used in high temperature operations, such as in a thermal recovery operation, may be made by introducing excess fiber length (EFL) to avoid tension on the one or more fibers 114 at high temperatures due to thermal expansion of the cable components. The fibers 114 generally take on a sinusoidal and/or helical form inside the cable 110. As the fibers 114 and the cable 110 are not retained relative to one another, the sinusoidal orientation of the fibers 114 within the cable 110 may result in intermittent contact points 116 and gaps 118 therebetween.
During a thermal recovery operation, the EFL of the cable 110 may be reduced and coatings on the one or more fibers 114 may adhere at contact points with each other or to an inner wall of the cable 110 either through partial melting or chemical bonding of the coatings. Applications using Weatherford's standard downhole cables above 300 degrees Celsius may be near the melting point of the polyimide coatings used on the fibers. At 350 degrees Celsius, the coating on the fibers may become brittle. After a thermal recovery operation, which may last over 30 days, when the cable temperature may then be reduced, the cable structure contracts and the fibers 114 within the cable 110 may return to a state of increased EFL. If the fibers 114 are adhered with each other or to an inner wall of the cable 110, the fibers 114 may not return to their respective original orientations and tight bends may form along the fibers resulting in increased optical loss.